Electrically programmable fuse elements are used in various electronic circuits including memory arrays, particularly in field programmable memory arrays. A field programmable memory array is essentially a read only memory which contains an array of electrically programmable fuse elements. These fuse elements have an initial impedance characteristic or state when manufactured, and can be irreversibly converted to have a second impedance characteristic by the application of a proper voltage and/or current through the fusing element. These memory arrays are field programmable in that a user can program the memory array rather than having programming information placed in the memory during the manufacturing process.
One type of electrically programmable fusing element is referred to as an antifuse and is described in U.S. Pat. No. 4,442,507 to Rosener. As manufactured the antifuse has a high resistance in parallel with a small capacitance. After being programmed (activated) the resistance changes to less than a hundred ohms. The antifuse typically can be activated by the application of approximately 10 volts at a current of 10 microamps applied for less than twenty nanoseconds.
However, in a typical memory array in which one terminal of the antifuse is floating and the other terminal of the antifuse is connected to a bit line, the combination of the voltage change on the bit line and the parasitic capacitance between the floating terminal of the antifuse and ground can provide conditions which may result in an inadvertent activation of the antifuse.
Therefore, it can be appreciated that a fuse protection circuit which protects an antifuse type of fusing element from being inadvertently activated and which occupies a small amount of chip area is highly desirable.